Do not retry an HTTP 408; lock retry cap and timeout values

The OpenAI and Anthropic adapters now report an HTTP 408 as
llm.TimeoutStatusError, which still matches llm.ErrTimeout but carries
its status. Retrying no longer retries any error carrying a 4xx status,
so a vendor's 408 is returned at once while a transport timeout, which
has no status, is still retried.

Tests: 408 (bare and wrapped) is not retried and a transport timeout is;
FromSettings integration cases for an OpenAI and Anthropic 408 and an
OpenAI transport timeout; the cap and default-timeout tests assert the
literal 20 s and 60 s instead of the constants they lock.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ai/llm/openai/openai.go

@@ -453,7 +453,7 @@ func (p *Provider) statusError(resp *http.Response, body []byte) error {
 	case http.StatusTooManyRequests:
 		return llm.NewRateLimitedError(retryAfter(resp.Header.Get("Retry-After"), time.Now()), p.apiKey)
 	case http.StatusRequestTimeout:
-		return fmt.Errorf("HTTP %d: %w", resp.StatusCode, llm.ErrTimeout)
+		return llm.NewTimeoutStatusError(resp.StatusCode, p.apiKey)
 	default:
 		return llm.NewProviderError(resp.StatusCode, body, p.apiKey)
 	}